The production of nuclear power and atomic weapons has created stockpiles of plutonium, uranium and other radioactive wastes throughout the United States and the world. Following irradiation, a large and growing quantity of nuclear fuels have been permanently withdrawn from nuclear reactors. Constituent elements of these spent nuclear fuels have not been separated by processing. The inherent toxicity, chemical and physical properties of the wastes, and potential use of the wastes in the production of nuclear weapons, creates unique and stringent demands for safe and effective long term disposal of the wastes. To effectively address these unique concerns, a final waste form which meets the highest standards of safety, security and accountability is required.
Contrasted against these criteria, the disposal of these wastes to date has often been entirely unsatisfactory. The wastes have been stored in temporary sites, have leaked from a variety of containers into the ground, or have otherwise been introduced into the environment. The introduction of these wastes into the ground has created an unfortunate situation wherein the material contaminates the surrounding soil, thereby greatly increasing the waste volume. Also, once in the ground, wastes are subject to transport via environmental mechanisms and thus threaten human health and safety as well as spread further contamination. Thus, it is desirable that the wastes be processed from their present form to a form suitable for long term storage. To insure an acceptable long term solution to the problem, the final waste form must be resistant to environmental transport and degradation, including attack by chemical and physical processes.
Wastes are presently commingled with a wide variety other constituents including corroded fuel, sludge, concrete grit, metal fragments, fuel containers, sand, soil, and dirt. Effective long term storage and immobilization of the wastes requires either that the wastes be removed from the environment for processing or that the wastes and other constituents together are converted into an acceptable waste form in situ. To convert the wastes to an acceptable waste form requires either that the wastes be separated from the other constituents and processed separately, or that the wastes together with the other constituents be processed together.
The separation of wastes from the other constituents presents a variety of technical challenges. Due to the radioactivity and toxicity of the wastes, separation can be both hazardous and expensive. Thus, to provide satisfactory and economical final disposal of these wastes, it is desirable that the wastes be processed into a final form without the hazardous and expensive step of removing the other constituents. It is also desirable that the wastes in their final form prevent removal of the fissile constituents of the wastes and immobilize the wastes to prevent degradation and transport of the wastes by environmental mechanisms.
Several methods for providing a final form for waste are known in the art. Vitrification to produce borosilicate glasses having waste constituents bound within the glasses has been shown as an effective method for treatment of low volume, high level wastes. In the vitrification process, wastes are mixed with glass-forming additives and converted into an amorphous glassy form by high temperature melting and cooling. The drawbacks of vitrification include the requirement that fluxing components such as alkalis, boron, or alkaline earth metals, must be added to the waste. These fluxing components are necessary in quantities sufficient to achieve viscosity for processing at temperatures low enough for practical application of the vitrification process. A further drawback of vitrification arises due to the low solubility of many of the waste components of interest in glass which prohibits large concentrations of the waste components in the final glass form. This low solubility greatly increases the required volume of the final waste form for a given volume of radioactive waste components of interest.
A final form for waste may also be accomplished by the incorporation of waste components of interest into synroc. The synroc process produces a crystalline final waste form and involves the steps of mixing precursors (oxide, hydroxide or sol-gel) with the wastes, calcinating the mixture at a temperature of about 750.degree. to 1100.degree. C. for about 1 to 16 hours, adding Ti powders to the mixture, cold pressing the mixture at a pressure of about 40 to 345 mpa, and hot pressing the mixture at a pressure of about 15 to 50 mpa and a temperature of about 1150.degree. to 1200.degree. C. Drawbacks of the synroc process include restrictions of both the valence and the size of waste ions which may be incorporated into the lattice of the final crystal waste form. To insure the correct valence and size of the waste ions requires significant pretreatment of the waste. Wastes present in metallic form must be first oxidized prior to synrock formation. Also, high temperatures and pressures must be utilized to successfully create a suitable final waste form.
Thus, there exists a need for an economical method for creating an acceptable final waste form for spent nuclear fuel and surplus plutonium without expensive pretreatment and oxidation of the waste.